Is modding a game piracy?

Solution
Yeah, generally speaking modding single player games is OK, but sometimes it's against the terms of service. It's not like you'll get arrested if you mod the game (or download mods for it), but if you decide to distribute mods, make sure that it's not forbidden.

A word of warning: While some developers are OK with modding, Steam may not be. A good example is the DSfix for Dark Souls. If you use it to set the fps limit to 60 (it's 30 by default) you'll be modifying the code of the game, which may lead to getting automatically banned from Steam.
Yeah, generally speaking modding single player games is OK, but sometimes it's against the terms of service. It's not like you'll get arrested if you mod the game (or download mods for it), but if you decide to distribute mods, make sure that it's not forbidden.

A word of warning: While some developers are OK with modding, Steam may not be. A good example is the DSfix for Dark Souls. If you use it to set the fps limit to 60 (it's 30 by default) you'll be modifying the code of the game, which may lead to getting automatically banned from Steam.
 
Solution